1. Field of the Invention
The present invention relates to an image heating apparatus suitable to be used as a thermal fixing apparatus mounted to an image forming apparatus such as a copier, a printer or the like, and more particularly, it relates to a heater having a heat generating resistor on a substrate, and an image heating apparatus mounting such a heater thereon.
2. Description of the Related Art
In the past, in image forming apparatuses such as electrophotographic copiers, laser beam printers and the like, a latent image corresponding to target image information was formed on an image bearing member by image forming process means, and, from the latent image, a visual image (toner image) was formed by using toner including resin having a thermally-soluble property. Then, the toner image was transferred onto a surface of a recording material such as a transferring paper, directly, or indirectly via an intermediate transferring member, thereby forming unfixed toner image on the surface of the recording material.
As a fixing apparatus for fixing the unfixed toner image onto the surface of the recording material as a permanent fixed image, a heating apparatus of heat roller type has generally been used. In such a heating apparatus, the toner image is fixed with heat and pressure by pinching and conveying the recording material as heated material by means of a fixing roller heated to a predetermined temperature and a pressure roller urged against the fixing roller.
In recent years, as described in Japanese Patent Application Laid-open No. S63-313182 (1988), a fixing apparatus of film heating type (referred to as “film heating fixing apparatus” hereinafter) for achieving power savings and for reducing time interval from ON of a power source to output of an image has been proposed and has been put to a practical use.
Such a film heating fixing apparatus comprises a heater unit as heating means, and pressurizing means such as a pressure roller (referred to merely as “pressure roller” hereinafter) for closely contacting a recording material against the heater unit. Further, the heater unit includes a heating member (referred to as “heater” hereinafter) fixedly supported, and a heat resistive film (referred to as “fixing film” hereinafter) as a flexible member conveyed while being urged against the heater. The film heating fixing apparatus serves to thermally fix the unfixed toner image formed on the surface of the recording material by applying heat from the heater to the recording material via the fixing film.
In the past, as an example of a heater attached to such a fixing apparatus, there has been proposed a heater 100 as shown in FIG. 12A. The heater 100 is fundamentally constituted by a substrate 101, and heat generating resistors 102 and 103 provided along a longitudinal direction of the substrate 101. A paper is generally conveyed along a shorter side of the substrate 101, and a longer side of the substrate is perpendicular to the conveyance direction. In this specification, a direction of the longer side is referred to as a longitudinal direction and a direction of the shorter side as the conveyance direction is referred to as a width-wise direction. In the drawings, a center of the longitudinal direction is designated by “E” and a center of the width-wise direction is designated by “F”. The heat generating resistors 102 and 103 are provided along the longitudinal direction of the substrate 101. By supplying power from power supplying electrodes 104, 105 and 106 electrically communicated with both ends of the heat generating resistors 102 and 103 to the heat generating resistors 102 and 103, the heat generating resistors 102 and 13 are heated.
As shown in a graph of FIG. 12B, each of the heat generating resistors 102 and 103 of the heater 100 has a resistance value per unit longitudinal length which is uniform along the longitudinal direction.
In the film heating fixing apparatus having the heater 100 in which the heat generating resistors 102 and 103 generate the uniform heat along the longitudinal direction, the heater 100 is closely contacted with the recording material via the fixing film having a low heat capacity, thereby transmitting the heat to the recording material. Thus, on-demand fixing is permitted. However, if recording materials having widths relatively smaller than the longitudinal lengths of the heat generating resistors 102 and 103 are conveyed continuously, a difference in surface temperature between a paper passing area and a paper non-passing area of the pressure roller with which the heater is closely contacted via the fixing film becomes greater, with the result that an outer diameter and coefficient of friction of the pressure roller may be changed in the longitudinal direction of the pressure roller.
To avoid this, a heater 100 which can treat various sizes of papers and which can suppress increase in temperature of the paper non-passing area has been proposed, as disclosed in Japanese Patent Application Laid-open No. H10-177319. This heater 100 has two kinds of heaters 107 and 108 having different heat generation distributions as shown in FIG. 13A, so that, when the print is performed on a small size paper, by increasing a power supplying rate to the heat generating resistor 107 having a high resistance value at a small size paper passing area thereof, the increase in temperature of a paper non-passing area thereof can be suppressed.
Further, in recent printers, it is requested to reduce a first printout time (FPOT) which is needed from a time when print command is sent to the printer to when a first recording paper is outputted. Thus, reduction in time needed rise-up the fixing apparatus to a fixing permitting temperature is also requested, and, to cope with this, an electric power instantaneously applied to the heater becomes very great. In this case, great stress acts on the substrate of the heater. As a heater design having a great resistance against the great stress, there has been proposed a heater design in which patterns of heat generating resistors are printed symmetrically with respect to the conveyance direction of the recording material and symmetrically with respect to left and right in the longitudinal direction (see Japanese Patent Application Laid-open No. 2006-004860, Japanese Patent Application Laid-open No. 2006-004861 and US-2005-0280682).
By the way, a technique in which a reinforcing layer is provided on a substrate to reinforce the substrate is also known (see Japanese Patent Application Laid-open No. H10-189218).
However, if the reinforcing layer is provided on the whole surface of the substrate, the cost will be increased.